These are just three recent examples of American teenagers lured from lives of relative comfort in the US to the front lines of the Islamic State in Iraq and Syria, or ISIS, a terrorist group known for its brutality - and increasingly, for its sophisticated recruitment campaigns that are drawing more and more disaffected teens from Western countries such as the US, the UK, and France.

According to CIA estimates, about 2,000 Westerners have traveled to Iraq and Syria (many via Turkey) to join ISIS. Of these, more than 100 have come from the US, at least 500 from the UK, and more than 700 from France, according to estimates from authorities in those countries.

Among the American teens were the trio of Colorado girls of Somali and Sudanese descent, aged 16, 17, and 18, who stole their passports and $2,000 in cash from their parents, skipped class, and boarded a flight to Turkey by way of Germany, where they were met by FBI officials after their parents discovered they were missing.

In April, 19-year-old American convert to Islam Shannon Conley, a nurse aid, was arrested at Denver International Airport attempting to board a flight to Turkey via Germany, where she was to marry an ISIS member. And earlier this month, suburban Chicago teen Mohammed Khan was arrested at O'Hare International Airport where he was planning to fly to Vienna and then onward to Istanbul, before crossing into Syria to join ISIS.

Why are growing numbers of American teens - and increasingly, teenage girls - drawn to leave the US and join ISIS, known for its brutal violence, executions, bombing, and beheading?

Largely, because ISIS is extremely good at luring Westerners into its ranks.

Here's how it preys on American youth.

Appeal to sense of identity

According to news and FBI reports, ISIS typically preys on Western youth who are disillusioned and have no sense of purpose or belonging.

Much like criminal gangs that offer a sense of family and belonging, ISIS offers disaffected teens a chance to join a group that gives them purpose and meaning - however misguided.

"The general picture provided by foreign fighters of their lives in Syria suggests camaraderie, good morale and purposeful activity, all mixed in with a sense of understated heroism, designed to attract their friends as well as to boost their own self-esteem," Richard Barrett of The Soufan Group wrote in a report called "Foreign fighters in Syria" this summer.

Sophisticated internet propaganda

In the world of terrorism, the Internet is a double-edged sword, used by authorities to track terrorists and would-be terrorists, as well as by terrorist groups to recruit fighters from around the world.

And according to Matthew Olsen, director of the National Counterterrorism Center, ISIS now operates the most sophisticate propaganda machine of any terrorist organization, as he told CNN in a recent piece on ISIS recruitment.

"Importantly, the group also views itself as the now-leader of a global jihadist movement," Olsen told CNN. "It turns out timely, high-quality media, and it uses social media to secure a widespread following."

Appeal to a sense of religious obligation

Muslim leaders worldwide have condemned ISIS, reiterating that there is no Islamic rationale for its actions and even launching a Twitter campaign against the group, #NotInMyName. Nonetheless, ISIS continues to appeal to a misguided sense of religious duty.

"They're often times searching for an identity, because what the jihadis are actually pushing is a specific narrative, which is: Your people (Muslims) are being oppressed in this place called Syria; your government is doing nothing; we're the only ones who are actually going to help you out," Aki Peritz, a former CIA officer, told Chicago's WGN network. "Why don't you join the fight?"

Female-targeted recruitment

Perhaps most surprising, ISIS is developing a new recruitment tactic: preying on teenage girls.

The terror group launched a new online propaganda campaign last week called "al-Zawra," which uses various social media platforms and videos as a sort of jihad primer for young women.

In one counter-propaganda effort in London, a group of activists staged a mock sex slave auction in the street to dramatize ISIS goals and treatment of women. "This is just an example of what is going on in Syria and Iraq in the places where the Islamic State has gained power," Karam Kruda, the man with the megaphone, says in the video.